Fusion has a notion of a "canvas color". In addition to the DoD, there is a color that is used for pixels that live outside of the DoD. It comes into play when compositing either smaller images, or with transforms that make some of the image not visible.

Normally, this canvas color is all black, with a 0 alpha, but some image operations will change it. For example, the color corrector's offset seems to change the color values, and inverting the alpha can result in the alpha being set to 1. I eventually discovered the "Set Canvas Color" node which can be used to set the canvas color back to black with 0 alpha (sometimes, you can also use a different edge mode).

What I haven't figured out is: is there a way to view what the canvas color is in a given image? Yesterday, I spent a good amount of time trying to figure out what had made an image's alpha become 1, and I still don't quite know where it is happening.